A Case of Advanced Extramammary Paget's Disease With a High Tumor Mutation Burden That Showed Partial Lymph Node Regression With Pembrolizumab.

Pembrolizumab has been found effective against various solid tumors with high tumor mutation burden, but there are no reports of successful treatment with pembrolizumab for extramammary Paget's disease (EMPD) with a high tumor mutation burden (TMB). This report describes a 71-year-old male patient who presented with irregularly shaped erythematous lesions on his scrotum, which had been there for several years. He was diagnosed with EMPD. A gene panel test indicated a high TMB. Six months after radical surgery, many lymph node metastases were found. We started a combination chemotherapy of tegafur-gimeracil-oteracil potassium (TS-1) and docetaxel. After three courses, a CT scan showed reductions in the sizes of all the metastatic lesions. However, the treatment was switched to pembrolizumab at the patient's request. After four cycles of the pembrolizumab treatment, a CT scan showed further shrinkage in most of the lymph node metastases. As far as we know, the present patient is the first case of advanced EMPD with response to pembrolizumab monotherapy.